Due to the prevalence of handheld electronic devices, such as smartphones and tablet PC, power supply is of vital importance. In addition to a main battery, a conventional handheld electronic device can be equipped with a backup battery for supplying the backup power required to shut down the handheld electronic device, store data therein, and prevent data loss which might otherwise occur while the main battery is being changed.
The conventional way of charging the backup battery entails charging the backup battery by a charging circuit (also known as a charger), such as a charging IC, which is different from the one usually used to charge the main battery. The conventional backup battery charger has its disadvantage, that is, it increases the required circuit area and incurs costs.